Audio signals, like speech or music, are encoded for example for enabling an efficient transmission or storage of the audio signals.
Audio encoders and decoders are used to represent audio based signals, such as music and background noise. These types of coders typically do not utilise a speech model for the coding process, rather they use processes for representing all types of audio signals, including speech.
Speech encoders and decoders (codecs) are usually optimised for speech signals, and can operate at either a fixed or variable bit rate.
An audio codec can also be configured to operate with varying bit rates. At lower bit rates, such an audio codec may work with speech signals at a coding rate equivalent to a pure speech codec. At higher bit rates, the audio codec may code any signal including music, background noise and speech, with higher quality and performance.
In stereo audio encoders, the received audio signal contains left and right channel audio signal information. Dependent on the available bit rate for transmission or storage different encoding schemes may be applied to the input channels. The left and right channels may be encoded independently, however there is typically correlation between the channels and many encoding schemes and decoders use this correlation to further reduce the bit rate required for transmission or storage of the audio signal.
Two commonly used stereo audio coding schemes are mid/side (MS) stereo encoding and intensity stereo (IS) stereo encoding. In MS stereo, the left and right channels are encoded into a sum and difference of the channel information signal. This encoding process therefore uses the correlation between the two channels to reduce the complexity with regard to the difference signal. In MS stereo, the coding and transformation is typically done both in frequency and time domains. MS stereo encoding has typically been used in high quality high bit rate stereophonic coding. MS coding however can not produce significantly compact coding for low bandwidth encoding.
IS coding, is preferred in mid-low bandwidth encoding scenarios. In IS coding a portion of the frequency spectra is coded using a mono encoder and the stereo image is reconstructed at the receiver/decoder by using scaling factors to separate the left and right channels.
IS coding produces a stereo encoded signal with typically lower stereo separation as the difference between the left and right channels is reflected by a gain factor only.
As is known in the art certain spectral frequencies are more significant with regards to the perception of the audio signal than others. Both MS and IS stereo encoding fails to use this information and does not encode the stereo signal optimally.